# Ultracold Quantum Gases Group

Welcome to the ultracold quantum gas research group at Aarhus University!

In our research we investigate the properties of atomic gases at extremely low temperatures. This allows us to understand the fundamental quantum mechanical behaviour of these many particle systems.

# News

## Observation of attractive and repulsive polarons in a Bose-Einstein condensate

The behavior of a mobile impurity particle interacting with a quantum-mechanical medium is of fundamental importance in physics. Due to the great flexibility of atomic gases, our understanding of the impurity problem has improved dramatically since it was realized experimentally in a particularly pure form using degenerate Fermi gases. However, there has not been such a realization of the impurity problem in a bosonic reservoir so far. Here, we use radio frequency spectroscopy of ultracold bosonic 39K atoms to experimentally demonstrate the existence of a well-defined quasiparticle state for an impurity interacting with a Bose-Einstein condensate (BEC). We measure the energy of the impurity both for attractive and repulsive interactions with the BEC, and find excellent agreement with theories that incorporate three-body correlations, both in the weak-coupling limits and across unitarity. Our results show that the spectral response consists of a well-defined quasiparticle peak at weak coupling and a continuum of excited many-body states. For increasing interaction strength, the spectrum is strongly broadened and becomes dominated by the many-body continuum, but no significant effects of three-body decay are observed. Our results open up intriguing prospects for studying mobile impurities in a bosonic environment, as well as strongly interacting Bose systems in general. http://arxiv.org/abs/1604.07883 (04/2016)

## Phase Separation and Dynamics of two-component Bose-Einstein condensates

The miscibility of two interacting quantum systems is an important testing ground for the understanding of complex quantum systems. Two-component Bose-Einstein condensates enable the investigation of this scenario in a particularly well controlled setting. In a homogeneous system, the transition between mixed and separated phases is fully characterised by a `miscibility parameter', based on the ratio of intra- to inter-species interaction strengths. Here we show, however, that this parameter is no longer the optimal one for trapped gases, for which the location of the phase boundary depends critically on atom numbers. We demonstrate how monitoring of damping rates and frequencies of dipole oscillations enables the experimental mapping of the phase diagram by numerical implementation of a fully self-consistent finite-temperature kinetic theory for binary condensates. The change in damping rate is explained in terms of surface oscillation in the immiscible regime, and counterflow instability in the miscible regime, with collisions becoming only important in the long time evolution. http://arxiv.org/abs/1604.08063 (04/2016)

## Preparation of ultracold atom clouds at the shot noise level

We prepare number stabilized ultracold clouds through the real-time analysis of non-destructive images and the application of feedback. In our experiments, the atom number N∼10^6 is determined by high precision Faraday imaging with uncertainty ΔN below the shot noise level, i.e., ΔN<√N. Based on this measurement, feedback is applied to reduce the atom number to a user-defined target, whereupon a second imaging series probes the number stabilized cloud. By this method, we show that the atom number in ultracold clouds can be prepared below the shot noise level. http://arxiv.org/abs/1604.05087 (04/2016)

## Absence of observable Efimov resonances in ultracold KRb mixtures

Ultracold atomic gases have recently become a driving force in few-body physics due to the observation of the Efimov effect. While initially observed in equal mass systems, one expects even richer few-body physics in the mass-imbalanced case. In previous experiments with ultracold mixtures of potassium and rubidium, an unexpected non-universal behavior of Efimov resonances was observed. In contrast, we measure the scattering length dependent three-body recombination coefficient in ultracold heteronuclear mixtures of 39K-87Rb and 41K-87Rb and do not observe any signatures of Efimov resonances for accessible scattering lengths in either mixture. Our results show excellent agreement with our theoretical model for the scattering dependent three-body recombination coefficient and reestablishes universality of the three-body parameter across isotopic mixtures. http://arxiv.org/abs/1604.03693 (04/2016)

## New grant: Experiments with Quantum Test Beds

Jan Arlt has recieved a grant from VILLUM FONDEN valued at DKK 4.7 million. View more details here. (01/2016)

## Congratulations to Romain!

On the 14^{th} of January, Romain C. G. F. Müller successfully defended his PhD thesis "A new experiment to probe and manipulate quantum systems". The assessment committee consisted of Prof. Leticia Tarruell from ICFO Barcelona and Prof. Henning Moritz from the University of Hamburg. He will carry on his scientific work by continuing in the group as a postdoctoral researcher.

scienceathome.org also did an interview with Romain. (01/2016)

## Time limited optimal dynamics beyond the Quantum Speed Limit

The quantum speed limit sets the minimum time required to transfer a quantum system completely into a given target state. At shorter times the higher operation speed results in a loss of fidelity. Here we quantify the trade-off between the fidelity and the duration in a system driven by a time-varying control. The problem is addressed in the framework of Hilbert space geometry offering an intuitive interpretation of optimal control algorithms. This approach leads to a necessary criterion for control optimality applicable as a measure of algorithm convergence. The time fidelity trade-off expressed in terms of the direct Hilbert velocity provides a robust prediction of the quantum speed limit and allows one to adapt the control optimization such that it yields a predefined fidelity. The results are verified numerically in a multilevel system with a constrained Hamiltonian and a classification scheme for the control sequences is proposed based on their optimizability. (12/2015)

## First paper from the MIX lab published!

**Tunable dual-species Bose-Einstein condensates of ^{39}K and ^{87}Rb**

We present the production of dual-species Bose-Einstein condensates of ^{39}K and ^{87}Rb. Preparation of both species in the |F=1,mF=−1> state enabled us to exploit a total of three Fesh\-bach resonances which allows for simultaneous Feshbach tuning of the ^{39}K intraspecies and the ^{39}K-^{87}Rb interspecies scattering length. Thus dual-species Bose-Einstein condensates were produced by sympathetic cooling of ^{39}K with ^{87}Rb. A dark spontaneous force optical trap was used for ^{87}Rb, to reduce the losses in ^{39}K due to light-assisted collisions in the optical trapping phase, which can be of benefit for other dual-species experiments. The tunability of the scattering length was used to perform precision spectroscopy of the interspecies Feshbach resonance located at 117.56(2)G and to determine the width of the resonance to 1.21(5)G by rethermalization measurements. The transition region from miscible to immiscible dual-species condensates was investigated and the interspecies background scattering length was determined to 28.5a0 using an empirical model. This paves the way for dual-species experiments with ^{39}K and ^{87}Rb BECs ranging from molecular physics to precision metrology. (11/2015)

## Nils wins Researchers Battle 2015

In a dissemitation contest with other PhD students from Aarhus University, Nils presented his reasearch in ultracold atoms for a general audience in just four minutes. Through a vote from the audience and three referees, he won the contest! Read more here where au.dk covered the event. (09/2015)

## Congratulations to Mirek!

On the 23^{rd} of September, Miroslav Gajdacz successfully defended his PhD thesis "Stabilizing production of atomic clouds". The assessment committee consisted of Prof. Dr. Christiane Koch from University of Kassel and Prof. David Guéry-Odelin from the University of Toulouse. He will carry on his scientific work by continuing in the group as a postdoctoral researcher. (09/2015)

## Congratulations to Lars!

On the 22^{nd} of September, Lars Johann Wacker successfully defended his PhD thesis "Few-body physics with ultracold potassium rubidium mixtures". The assessment committee consisted of Prof. Selim Jochim from Heidelberg University and Dr. Steven Knoop from the University of Amsterdam. He will carry on his scientific work by continuing in the group as a postdoctoral researcher. (09/2015)

## Congratulations to Danny!

Danny Birkmose succesfully defended his Master thesis "Investigation of Dual Species Bose Einstein Condensates". Danny has been working as a master student on the MIX experiment for the last year. (08/2015)

## A continuously pumped reservoir of ultracold atoms

Typical sources of ultracold atoms operate with a considerable delay between the delivery of ensembles due to sequential trapping and cooling schemes. Therefore, alternative schemes for the continuous generation of ultracold atoms are highly desirable. Here, we demonstrate the continuous loading of a magnetic trap from a quasi-continuous atom beam. We achieve a steady state with 3.8x10^{7} magnetically trapped atoms and a temperature of 102uK. The ensemble is protected from laser light sources, a requirement for its application in metrological tasks or sympathetic cooling. The continuous scheme is robust and applicable to a wide range of particles and trapping potentials.

In addition, this work received media attention at 2physics and at Physicsworld.com. The paper can be found here. (07/2015)

## New grant: Few-body Quantum Physics with Ultracold Atoms

Jan Arlt has recieved a grant from The Danish Council for Independent Research valued at DKK 2.6 million. View more details here (in danish). (05/2015)

# Funding

**Our research is supported by:**

The Danish National Research Foundation within the Center for Quantum Optics (QUANTOP).